Re: [PATCH 2/2] alpha: add a delay before serial port read

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, May 06, 2020 at 07:23:31AM -0400, Mikulas Patocka wrote:
> The patch 92d7223a74235054f2aa7227d207d9c57f84dca0 ("alpha: io: reorder
> barriers to guarantee writeX() and iowriteX() ordering #2") broke boot on
> the Alpha Avanti platform.
> 
> The patch changes timing between accesses to the ISA bus, in particular,
> it reduces the time between "write" access and a subsequent "read" access.
> 
> This causes lock-up when accessing the real time clock and serial ports.
> 
> This patch fixes the serial ports by adding a small delay before the "inb"
> instruction.
> 
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> Fixes: 92d7223a7423 ("alpha: io: reorder barriers to guarantee writeX() and iowriteX() ordering #2")
> Cc: stable@vger.kernel.org	# v4.17+
> 
> ---
>  drivers/tty/serial/8250/8250_port.c |    4 ++++
>  1 file changed, 4 insertions(+)
> 
> Index: linux-stable/drivers/tty/serial/8250/8250_port.c
> ===================================================================
> --- linux-stable.orig/drivers/tty/serial/8250/8250_port.c	2020-05-06 08:25:19.000000000 +0200
> +++ linux-stable/drivers/tty/serial/8250/8250_port.c	2020-05-06 09:04:17.000000000 +0200
> @@ -442,6 +442,10 @@ static unsigned int mem32be_serial_in(st
>  
>  static unsigned int io_serial_in(struct uart_port *p, int offset)
>  {
> +#ifdef CONFIG_ALPHA
> +/* we need a small delay, the Alpha Avanti chipset locks up with back-to-back accesses */
> +	ndelay(300);
> +#endif

We really do not like #ifdef in .c files, especially ones that cause a
coding style violation :)

Why can't you do this as a quirk for this specific chipset?  You should
tie it to the serial port hardware type, not to the CPU type.

thanks,

greg k-h